Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

From: Will Deacon <will@kernel.org>
To: "Aiqun(Maria) Yu" <quic_aiquny@quicinc.com>
Cc: corbet@lwn.net, catalin.marinas@arm.com, maz@kernel.org,
	quic_pkondeti@quicinc.com, quic_kaushalk@quicinc.com,
	quic_satyap@quicinc.com, quic_shashim@quicinc.com,
	quic_songxue@quicinc.com, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option
Date: Tue, 11 Jul 2023 09:22:31 +0100	[thread overview]
Message-ID: <20230711082226.GA1554@willie-the-truck> (raw)
In-Reply-To: <5cf15f85-0397-96f7-4110-13494551b53b@quicinc.com>

On Tue, Jul 11, 2023 at 12:02:22PM +0800, Aiqun(Maria) Yu wrote:
> On 7/10/2023 5:37 PM, Will Deacon wrote:
> > On Mon, Jul 10, 2023 at 01:59:55PM +0800, Maria Yu wrote:
> > > In order to be able to disable lse_atomic even if cpu
> > > support it, most likely because of memory controller
> > > cannot deal with the lse atomic instructions, use a
> > > new idreg override to deal with it.
> > 
> > This should not be a problem for cacheable memory though, right?
> > 
> > Given that Linux does not issue atomic operations to non-cacheable mappings,
> > I'm struggling to see why there's a problem here.
> 
> The lse atomic operation can be issued on non-cacheable mappings as well.
> Even if it is cached data, with different CPUECTLR_EL1 setting, it can also
> do far lse atomic operations.

Please can you point me to the place in the kernel sources where this
happens? The architecture doesn't guarantee that atomics to non-cacheable
mappings will work, see "B2.2.6 Possible implementation restrictions on
using atomic instructions". Linux, therefore, doesn't issue atomics
to non-cacheable memory.

> > Please can you explain the problem that you are trying to solve?
> 
> In our current case, it is a 100% reproducible issue that happened for
> uncached data, the cpu which support LSE atomic, but the system's DDR
> subsystem is not support this and caused a NOC error and thus synchronous
> external abort happened.

So? The Arm ARM allows this behaviour and Linux shouldn't run into it.

Will

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